\begin{table}%t9
\caption{Comparison of average values of colour gradients for the subsample 
$IR_0$ of objects with zero IRAS 100~$\mu$ flux and the subsample~$IR_1$ of objects 
with non zero flux (Knapp et~al. \cite{Knapp89}).\label{tab9} }
%\centerline
{
\begin{tabular}{lllllll}
\hline  \hline    
  $Y$          &$IR_0$& $N$ &\multicolumn{1}{c}{$Av_0$} &$IR_1$& $N$ &  \\
  \hline
$\Delta_{BV}$&$IR_0$& 17&$-0.065\pm.005$&$IR_1$& 35&$-0.067\pm.004$ \\
$\Delta_{UB}$& id   & 13&$-0.166\pm.015$& id   & 27&$-0.152\pm.007$ \\
$\Delta_{BR}$& id   & 14&$-0.083\pm.008$& id   & 28&$-0.081\pm.005$ \\
$\Delta_{VI}$& id   & 15&$-0.057\pm.009$& id   & 34&$-0.063\pm.006$ \\
$\Delta_{VJ}$& id   & 17& $-0.106\pm.026$& id & 35&$-0.147\pm.015$  \\
id           & id   & 16& $-0.089\pm.014$& id & 34&$-0.139\pm.013$  \\
$\Delta_{VK}$& id   & 17& $-0.199\pm.029$& id & 35&$-0.237\pm.018$  \\
id           & id   & 16& $-0.175\pm.017$& id & 34&$-0.227\pm.015$  \\
$\Delta_{JK}$& id   & 17& $-0.090\pm.009$& id & 35&$-0.092\pm.005$  \\
\hline
\end{tabular}}
\smallskip
1) Gradient $Y$; (2) subsample $IR_0$; (3) 
$N$ number of objects; (4) average $Av_0$; (5) subsample $IR_1$; (6) $N$ number of objects; 
(7) average $Av_1$. NB: the 2nd. line for $\Delta_{VJ}$, $\Delta_{VK}$ gives the 
solution after the rejection at $3\sigma$ of NGC~4660 from the subsample $IR_0$ 
and~5866 from $IR_1$.
\end{table}